The clinical interface between immunology, hematology, and oncology has long been appreciated. Many conditions treated by the hematologist or oncologist have either an autoimmune or immunodeficient component to their pathophysiology that has led to the widespread adoption of immunosuppressive medications by hematologists, whereas oncologists have sought immunologic adjuvants that might enhance endogenous immunity to tumors. To date, these interventions have generally consisted of nonspecific modes of immunosuppression and immune stimulation. In addition to the limited efficacy of these interventions, toxicities secondary to their nonspecificity have also limited their overall success. Therefore, alternative strategies have been sought.
Elucidation of the functional role of a rapidly increasing number of cell surface molecules has contributed greatly to the integration of immunology with clinical hematology and oncology. Nearly 200 cell surface antigens have been identified on cells of the immune and hematopoietic systems (Schlossman S F, Boumsell L, Gilks J M, Harlan T, Kishimoto, C Morimoto C, Ritz J., Shaw S, Silverstein R L, Springer T A, Tedder T F, Todd RF:CD antigens (1993), Blood 83:879, 1994). These antigens represent both lineage-restricted and more widely distributed molecules involved in a variety of processes, including cellular recognition, adhesion, induction and maintenance of proliferation, cytokine secretion, effector function, and even cell death. Recognition of the functional attributes of these molecules has fostered novel attempts to manipulate the immune response. Although molecules involved in cellular adhesion and antigen-specific recognition have previously been evaluated as targets of therapeutic immunologic intervention, recent attention has focused on a subgroup of cell surface molecules termed co-stimulatory molecules (Bretscher P: “The two-signal model of lymphocyte activation twenty-one years later.”Immunol. Today 13:73, (1992); Jenkins M K, Johnson J G: “Molecules involved in T-cell co-stimulation.” Curr Opin Immunol 5:351, (1993); Geppert T, Davis L. Gur H. Wacholtz M. Lipsky P: “Accessory cell signals involved in T-cell activation.” Immunol Rev 117:5, (1990); Weaver C T, Unanue E R: “The co-stimulatory function of antigen-presenting cells.” Immunol Today 11:49, (1990); Stennam R M, Young J W: “Signals arising from antigen-presenting cells.” Curr Opin Immunol 3:361, (1991)). Co-stimulatory molecules do not initiate but rather enable the generation and amplification of antigen-specific T-cell responses and effector function (Bretscher P: “The two-signal model of lymphocyte activation twenty-one years later.” Immunol. Today 13:73, (1992); Jenkins M K, Johnson J G: “Molecules involved in T-cell co-stimulation.” Curr Opin Immunol 5:351, (1993); Geppert T, Davis L. Gur H. Wacholtz M. Lipsky P: “Accessory cell signals involved in T-cell activation.” Immunol Rev 117:5, (1990); Weaver C T, Unanue E R: “The co-stimulatory function of antigen-presenting cells.” Immunol Today 11:49, (1990); Stennam R M, Young J W: “Signals arising from antigen-presenting cells.” Curr Opin Immunol 3:361, (1991); June C H, Bluestone J A, Linsley P S, Thompson C D: “Role of the CD28 receptor in T-cell activation.” Immunol Today 15:321, (1994)).
Recently, one specific co-stimulatory pathway termed B7:CD28 has been studied by different research groups because of its significant role in B and T cell activation (June C H, Bluestone J A, Linsley P S, Thompson C D: “Role of the CD28 receptor in T-cell activation.”Immunol Today 15:321, (1994); June C H, Ledbetter J A: “The role of the CD28 receptor during T-cell responses to antigen.” Annu Rev Immunol 11:191, (1993); Schwartz R H: “Co-stimulation of T lymphocytes: The role of CD28, CTLA-4, and B7/BB1 in interleukin-2 production and immunotherapy.” Cell 71:1065–1068, (1992); Jenkins M K, Taylor P S, Norton S D, Urdahl K B: “CD28 delivers a costimulatory signal involved in antigen-specific IL-2 production by human T cells.” Journal of Immunology 147:2461–2466 (1991)). Since this ligand:receptor pathway was discovered four years ago, a large body of evidence has accumulated suggesting that B7:CD28 interactions represent one of the critical junctures in determining immune reactivity versus anergy (June C H, Bluestone J A, Linsley P S, Thompson C D: “Role of the CD28 receptor in T-cell activation.” Immunol Today 15:321, (1994); June C H, Ledbetter J A: “The role of the CD28 receptor during T-cell responses to antigen.” Annu Rev Immunol 11:191, (1993); Schwartz RH: “Co-stimulation of T lymphocytes: The role of CD28, CTLA-4, and B7/BB1 in interleukin-2 production and immunotherapy.” Cell 71:1065–1068, (1992); Cohen J: “Mounting a targeted strike on unwanted immune responses” (news; comment). Science 257:751, (1992); Cohen J: “New protein steals the show as ‘co-stimulator’ of T cells” (news; comment). Science 262:844, (1993)).
In particular, the role of the human B7 antigens, i.e., human B7.1 (CD80) and B7.2 (CD86), has been reported to play a co-stimulatory role in T-cell activation. See, e.g., Gimmi C D, Freeman, G J, Gribben J G, Sugita K, Freedman A S, Morimoto C, Nadler L M: “B-cell surface antigen B7 provides a costimulatory signal that induces T cells to proliferate and secrete interleukin 2.” Proc. Natl. Acad. Sci. (USA) 88:6575–6579 (1991).
1. B7.1 and B7.2 Co-stimulatory Role in T Cell Activation
The elaboration of a successful immune response depends on a series of specific interactions between a T cell and an antigen presenting cell. Although the essential first step in this process depends upon the binding of antigen to the T cell receptor, in the context of the MHC class II molecule (Lane, P. J. L., F. M. McConnell, G. L. Schieven, E. A. Clark, and J. A. Ledbetter, (1990), “The Role of Class II Molecules in Human B Cell Activation.” The Journal of Immunology, 144:3684–3692), this interaction alone is not sufficient to induce all the events necessary for a sustained response to a given antigen (Schwartz, R. H. (1990), “A Cell Culture Model for T Lymphocyte Clonal Anergy.”Science, 248:1349; Jenkins, M. K. (1992), “The Role of Cell Division in the Induction of Clonal Anergy.”Immunology Today, 13:69; Azuma, M., M. Cayabyab, D. Buck, J. H. Phillips, and L. L. Lanier, (1992), “Involvement of CD28 in MHC-unrestricted Cytotoxicity Mediated by a Human Natural Killer Leukemia Cell Line.”The Journal of Immunology, 149:1115–1123; Azuma, M., M. Cayabyab, D. Buck, J. H. Phillips, and L. L. Lanier, (1992), “CD28 Interaction with B7 Costimulates Primary Allogeneic Proliferative Responses and Cytotoxicity Mediated by Small Resting T Lymphocytes.” J. Exp. Med., 175:353–360); S. D. Norton, L. Zuckerman, K. B. Urdahl, R. Shefner, J. Miller, and M. K. Jenkins, (1992), “The CD28 Ligand, B7, Enhances IL-2 Production by Providing a Costimulatory Signal to T Cells.” The Journal of Immunology, 149:1556–1561; R. H. Schwartz, (1992), “Costimulation of T Lymphocytes: The Role of CD28, CTLA-4, and B7/BB1 in Interleukin-2 Production and Immunotherapy.” Cell 71:1065–1068).
The involvement of certain other co-stimulatory molecules is necessary (Norton, S. D., L. Zuckerman, K. B. Urdahl, R. Shefner, J. Miller, and M. K. Jenkins, (1992), “The CD28 Ligand, B7, Enhances IL-2 Production by Providing A Costimulatory Signal to T Cells.” The Journal of Immunology, 149:1556–1561). “The homodimers CD28 and CTLA-4 expressed on T cells” (June, C. H., J. A. Ledbetter, P.S. Linsley, and C. B. Thompson, (1990), “Role of the CD28 Receptor in T-Cell Activation.”Immunology Today, 11:211–216; Linsley, P. S., W. Brady, M. Urnes, L. S. Grosmaire, N. K. Damle, and J. A. Ledbetter, (1991), “CTLA-4 is a Second Receptor for the B Cell Activation Antigen B7.” J. Exp. Med., 174:561), together with B7.1 (CD80) and B7.2 (CD86) expressed on antigen presenting cells, are major pairs of co-stimulatory molecules necessary for a sustained immune response (Azuma, M., H. Yssel, J. H. Phillips, H. Spits, and L. L. Lanier, (1993), “Functional Expression of B7/BB1 on Activated T Lymphocytes.” J. Exp. Med., 177:845–850; Freeman, G. J., A. S. Freedman, J. M. Segil, G. Lee, J. F. Whitman, and L M. Nadler, (1989), “B7, A New Member of the Ig Superfamily with Unique Expression on Activated and Neoplastic B Cells.” The Journal of Immunology, 143:2714–2722; Hathcock, K. S., G. Laslo, H. B. Dickler, J. Bradshaw, P. Linsley, and R. J. Hodes, (1993), “Identification of an Alternative CTLA-4 Ligand Costimulatory for T Cell Activation.” Science, 262:905–911; Hart, D. N. J., G. C. Starling, V. L. Calder, and N. S. Fernando, (1993). “B7/BB-1 is a Leucocyte Differentiation Antigen on Human Dendritic Cells Induced by Activation.” Immunology, 79:616–620). It can be shown in vitro that the absence of these co-stimulatory signals leads to an aborted T cell activation pathway and the development of unresponsiveness to the specific antigen, or anergy. (See, e.g., Harding, F. A., J. G. McArthur, J. A. Gross, D. M. Raulet, and J. P. Allison, (1992), “CD28 Mediated Signalling Co-simulates Murine T Cells and Prevents Induction of Anergy in T Cell Clones.” Nature, 356:607–609; Gimmi, C. D., G. J. Freeman, J. G. Gribben, G. Gray, and L. M. Nadler, (1993); “Human T-Cell Clonal Anergy is Induced by Antigen Presentation in the Absence of B7 Costimulation.” Proc. Natl. Acad. Sci., 90:6586–6590; Tan, P., C. Anasefti, J. A. Hansen, J. Melrose, M. Brunvand, J. Bradshaw, J. A. Ledbetter, and P.S. Linsley, (1993), “Induction of Alloantigen-specific Hyporesponsiveness in Human T Lymphocytes by Blocking Interaction of CD28 with Its Natural Ligand B7/BB1.” J. Exp. Med., 177:165–173). Achievement of in vivo tolerance constitutes a mechanism for immunosuppression and a viable therapy for organ transplant rejection and for the treatment of autoimmune diseases. This has been achieved in experimental models following the administration of CTLA-4Ig (Lenschow, D. J., Y. Zeng, R. J. Thistlethwaite, A. Montag, W. Brady, M. G. Gibson, P. S. Linsley, and J. A. Bluestone, (1992), “Long-Term Survival of Xenogeneic Pancreatic Islet Grafts Induced by CTLA-4Ig.” Science, 257:789–795).
The molecules B7.1 and B7.2 can bind to either CD28 or CTLA-4, although B7.1 binds to CD28 with a Kd of 200 Nm and to CTLA-4 with a 20-fold higher affinity (Linsley, P. S., E. A. Clark, and J. A. Ledbetter, (1990), “T-Cell Antigen CD28 Mediates Adhesion with B Cells by Interacting with Activation Antigen B7/BB-1.” Proc. Natl. Acad. Sci., 87:5031–5035; Linsley et al, (1993), “The Role of the CD28 receptor during T cell responses to antigen,” Annu. Rev. Immunol., 11:191–192; Linesley et al, (1993), “CD28 Engagement by B7/BB-1 Induces Transient Down-Regulation of CD28 Synthesis and Prolonged Unresponsiveness to CD28 Signaling,” The Journal of Immunology, 150:3151–3169). B7.1 is expressed on activated B cells and interferon induced monocytes, but not resting B cells (Freeman, G. J., G. S. Gray, C. D. Gimmi, D. B. Lomarrd, L-J. Zhou, M. White, J. D. Fingeroth, J. G. Gribben, and L M. Nadler, (1991). “Structure, Expression and T Cell Costimulatory Activity of the Murine Homologue of the Human B Lymphocyte Activation Antigen B7,” J. Exp. Med., 174:625–631). B7.2, on the other hand, is constitutively expressed at very low levels on resting monocytes, dendritic cells and B cells, and its expression is enhanced on activated T cells, NK cells and B lymphocytes (Azuma, M. D. Ito, H. Yagita, K. Okumura, J. H. Phillips, L. L. Lanier, and C. Somoza, 1993, “B70 Antigen is a Second Ligand for CTLA-4 and CD28,” Nature, 366:76–79). Although B7.1 and B7.2 can be expressed on the same cell type, their expression on B cells occurs with different kinetics (Lenschow, D. J., G. H. Su, L. A. Zuckerman, N. Nabavi, C. L. Jellis, G. S. Gray, J. Miller, and J. A. Bluestone, (1993), “Expression and Functional Significance of an Additional Ligand for CTLA-4,” Proc. Natl. Acad. Sci., USA, 90:11054–11058; Boussiotis, V. A., G. J. Freeman, J. G. Gribben, J. Daley, G. Gray, and L. M. Nadler, (1993), “Activated Human B Lymphocytes Express Three CTLA-4 Counter-receptors that Co-stimulate T-Cell Activation.” Proc. Natl. Acad. Sci., USA, 90:11059–11063). Further analysis at the RNA level has demonstrated that B7.2 mRNA is constitutively expressed, whereas B7.1 MRNA is detected 4 hours after activation and initial low levels of B7.1 protein are not detectable until 24 hours after stimulation (Boussiotis, V. A., G. J. Freeman, J. G. Gribben, J. Daley, G. Gray, and L. M. Nadler, (1993), “Activated Human B Lymphocytes Express Three CTLA-4 Counter-receptors that Co-stimulate T-Cell Activation,” Proc. Natl. Acad. Sci., USA, 90:11059–11063). CTLA-4/CD28 counter receptors, therefore, may be expressed at various times after B Cell activation.
More recently, it has been suggested that the second T cell associated co-receptor CTLA-4 apparently functions as a negative modulator to override and prevent a runaway immune system (Krummel M, Allison J: “CD28 and CTLA-4 have opposing effects on the response of T cells to stimulation.” J. Exp. Med. 182:459–466 (1995)). The CTLA-4 receptor plays a critical role in down regulating the immune response, as evidenced in CTLA-4 knockout mice. Knockout mice born without the ability to express the CTLA-4 gene die within 3–4 weeks of severe lymphoproliferative disorder (Tivol E A, Borriello G, Schweitzer A N, Lynch W P, Bluestone J A, Sharpe A H: “Loss of CTLA-4 leads to massive lymphoproliferation and fatal multiorgan tissue destruction, revealing a critical negative regulatory role of CTLA-4.” Immunity 3:541–547 (1995)). CTLA-4 is thought to function through signaling mechanisms linked to induction of apoptosis (Gribben J G, Freeman G J, Boussiotis V A, Rennert P, Jellis C L, Greenfield E, Barber M, Restivo Jr. V A, Ke X, Gray G S, Nadler L M: “CTLA-4 mediates antigen specific apoptosis of human T cells.” Proc. Natl. Acad. Sci. (USA) 92:811–815 (1995)), triggered through as yet undefined ligand binding to specific cites on the receptor. It has been shown in vitro that the blocking of the B7.1/B7.2 dependent co-stimulatory signals in various ways leads to an aborted T cell activating pathway and the development of unresponsiveness to the specific antigen (Lederman S, Chess L, Yellin M J: “Murine monoclonal antibody (5c8) recognizes a human glycoprotein on the surface of T-lymphocytes, compositions containing same.” U.S. Pat. No. 5,474,771 (Dec. 12, 1995); Linsley P S, Ledbetter J A, Damle N K, Brady W: “Chimeric CTLA4 receptor and methods for its use.” U.S. Pat. No. 5,434,131 (Jul. 18, 1995); Harding, 1992; Gimmi C D, Freeman G J, Bribben J G, Gray G, Nadler L M: “Human T-cell clonal anergy is induced by antigen presentation in the absence of B7 costimulation.” Proc. Natl. Acad. Sci. (USA) 90:6586–6590 (1993); Tan P, Anasetti C, Hansen J A, Melrose J, Brunvand M, Bradshaw J, Ledbetter J A, Linsley P S: “Induction of alloantigen-specific hyporesponsiveness in human T lymphocytes by blocking interaction of CD28 with its natural ligand B7/BB1.” J. Exp. Med. 177:165–173 (1993)). Achievement of in vivo tolerance, anergy, or depleting of antigen-specific T cells would constitute a mechanism for immunosuppression and a viable therapy for organ transplant rejection or plausible treatment for autoimmune diseases.
The differential temporal expression of B7.1 and B7.2 suggests that the interaction of these two molecules with CTLA-4 and/or CD28 deliver distinct but related signals to the T cell (LaSalle, J. M., P. J. Tolentino, G. J. Freeman, L. M. Nadler, and D. A. Hafler, (1992), “CD28 and T Cell Antigen Receptor Signal Transduction Coordinately Regulate Interleukin 2 Gene Expression In Response to Superantigen Stimulation,” J. Exp. Med., 176:177–186; Vandenberghe, P., G. J. Freeman, L. M. Nadler, M. C. Fletcher, M. Kamoun, L. A. Turka, J. A. Ledbetter, C. B. Thompson, and C. H. June, (1992), “Antibody and B7/BB1-mediated Ligation of the CD28 Receptor Induces Tyrosine Phosphorylation in Human T Cells,” The Journal of Experimental Medicine, 175:951–960). The exact signaling functions of CTLA-4 and CD28 on the T cell are currently unknown (Janeway, C. A., Jr. and K. Bottomly, (1994), “Signals and Signs for Lymphocyte Responses,” Cell, 76.275285). However, it is possible that one set of receptors could provide the initial stimulus for T cell activation and the second, a sustained signal to allow further elaboration of the pathway and clonal expansion to take place (Linsley, P. S., J. L. Greene, P. Tan, J. Bradshaw, J. A. Ledbetter, C. Anasetti, and N. K. Damle, (1992), “Coexpression and Functional Cooperation of CTLA-4 and CD28 on Activated T Lymphocytes,” J. Exp. Med., 176:1595–1604). The current data supports the two-signal hypothesis proposed by Jenkins and Schwartz (Schwartz, R. H., (1990), “A Cell Culture Model for T Lymphocyte Clonal Anergy,” Science, 248:1349; Jenkins, M. K., (1992), “The Role of Cell Division in the Induction of Clonal Anergy,” Immunology Today, 13:69) that both a TCR and co-stimulatory signal are necessary for T cell expansion, lymphokine secretion and the full development of effector function (Greenan, V. and G. Kroemer, (1993), “Multiple Ways to Cellular Immune Tolerance,” Immunology Today, 14:573). The failure to deliver the second signal results in the inability of T cells to secrete IL-2 and renders the cell unresponsive to antigen.
Structurally, both B7.1 and B7.2 contain extracellular immunoglobulin superfamily V and C-like domains, a hydrophobic transmembrane region and a cytoplasmic tail (Freeman, G. J., J. G. Gribben, V. A. Boussiotis, J. W. Ng, V. Restivo, Jr., L. A. Lombard, G. S. Gray, and L. M. Nadler, (1993), “Cloning of B7.2: A CTLA-4 Counter-receptor that Co-stimulates Human T Cell Proliferation,” Science, 262:909). Both B7.1 and B7.2 are heavily glycosylated. B7.1 is a 44–54 kD glycoprotein comprised of a 223 amino acid extracellular domain, a 23 amino acid transmembrane domain, and a 61 amino acid cytoplasmic tail. B7.1 contains 3 potential protein kinase phosphorylation sites. (Azuma, M., H. Yssel, J. H. Phillips, H. Spits, and L. L. Lanier, (1993), “Functional Expression of B7/BB1 on Activated T Lymphocytes,” J. Exp. Med., 177:845–850). B7.2 is a 306 amino acid membrane glycoprotein. It consists of a 220 amino acid extracellular region, a 23 amino acid hydrophobic transmembrane domain and a 60 amino acid cytoplasmic tail (Freeman, G. J., A. S. Freedman, J. M. Segil, G. Lee, J. F. Whitman, and L M. Nadler, (1989), “B7, A New Member of the Ig Superfamily with Unique Expression on Activated and Neoplastic B Cells,” The Journal of Immunology, 143:2714–2722). Although both B7.1 and B7.2 genes are localized in the same chromosomal region (Freeman, G. J., D. B. Lombard, C. D. Gimmi, S. A. Brod, L Lee, J. C. Laning, D. A. Hafler, M. E. Dorf, G. S. Gray, H. Reiser, C. H. June, C. B. Thompson, and L. M. Nadler, (1992), “CTLA-4 and CD28 MRNA are Coexpressed in Most T Cells After Activation,” The Journal of Immunology, 149:3795–3801; Schwartz, R. H., (1992), “Costimulation of T Lymphocytes: The Role of CD28, CTLA-4, and B7/BB1” in Selvakumar, A., B. K. Mohanraj, R. L. Eddy, T. B. Shows, P. C. White, C. Perrin, and B. Dupont, (1992), “Genomic Organization and Chromosomal Location of the Human Gene Encoding the B-Lymphocyte Activation Antigen B7,” Immunogenetics, 36:175–181), these antigens do not share a high level of homology. The overall homology between B7.1 and B7.2 is 26% and between murine B7.1 and human B7.1 is 27% (Azuma, M., H. Yssel, J. H. Phillips, H. Spits, and L. L. Lanier, (1993), “Functional Expression of B7/BB1 on Activated T Lymphocytes,” J. Exp. Med., 177:845–850; Freeman, G. J., A. S. Freedman, J. M. Segil, G. Lee, J. F. Whitman, and L M. Nadler, (1989), “B7, A New Member of the Ig Superfamily with Unique Expression on Activated and Neoplastic B Cells,” The Journal of Immunology, 143:2714–2722). Although alignment of human B7.1 human B7.2 and murine B7.1 sequences shows few stretches of lengthy homology, it is known that all three molecules bind to human CTLA-4 and CD28. Thus, there is most likely a common, or closely homologous region shared by the three molecules that may be either contiguous or conformational. This region may constitute the binding site of the B7.1 and B7.2 molecules to their counter-receptors. Antibodies raised against these epitopes could potentially inhibit the interaction of B7 with its counter-receptor on the T cell. Furthermore, antibodies that cross-reacted with this region on both B7.1 and B7.2 molecules would potentially have practical advantages over antibodies directed against B7.1 or B7.2 separately.
2. Blockade of the B7/CD28 Interaction
Blocking of the B7/CD28 interaction offers the possibility of inducing specific immunosuppression, with potential for generating long lasting antigen-specific therapeutic effects. Antibodies or agents that temporarily prevent this interaction may be useful, specific and safe clinical immunosuppressive agents, with potential for generating long term antigen-specific therapeutic effects.
Antibodies to either B7.1 or B7.2 have been shown to block T cell activation, as measured by the inhibition of IL-2 production in vitro (DeBoer, M., P. Parren, J. Dove, F. Ossendorp, G. van der Horst, and J. Reeder, (1992), “Functional Characterization of a Novel Anti-B7 Monoclonal Antibody,” Eur. Journal of Immunology, 22:3071–3075; Azuma, M., H. Yssel, J. H. Phillips, H. Spits, and L. L. Lanier, (1993), “Functional Expression of B7/BB1 on Activated T Lymphocytes,” J. Exp. Med., 177:845–850). However, different antibodies have been shown to vary in their immunosuppressive potency, which may reflect either their affinity or epitope specificity. A possible explanation for this may reside in the ability of some antibodies to block only the binding of B7 to CD28, while promoting apoptosis or some other form of negative signaling through the CTLA-4 receptor in activated T cells. Some antibodies to B7.1 or B7.2 may, in fact, hinder the activity of CTLA-4 by cross-reacting with the CTLA-4 binding domain. CTLA-4Ig fusion protein and anti-CD28 Fabs were shown to have similar effects on the down regulation of IL-2 production.
In vivo administration of a soluble CTLA-4Ig fusion protein has been shown to suppress T cell dependent antibody responses in mice (Linsley, P. S., J. L. Greene, P. Tan, J. Bradshaw, J. A. Ledbetter, C. Anasetti, and N. K. Damle, (1992), “Coexpression and Functional Cooperation of CTLA-4 and CD28 on Activated T Lymphocytes,” J. Exp. Med., 176:1595–1604; Lin, H., S. F. Builing, P. S. Linsley, R. O. Wei, C. D. Thompson, and L. A. Turka, (1993), “Long-term Acceptance of Major Histocompatibility Complex Mismatched Cardiac Allografts Induced by CTLA-4-Ig Plus Donor Specific Transfusion,” J. Exp. Med., 178:1801) and, furthermore, larger doses were also able to suppress responses to a second immunization, demonstrating the feasibility of this approach for the treatment of antibody mediated autoimmune disease. In addition, CTLA-4Ig was able to prevent pancreatic islet cell rejection in mice by directly inhibiting the interaction of T cells and B7.1/B7.2 antigen presenting cells (Lenschow, D. J., G. H. Su, L. A. Zuckerman, N. Nabavi, C. L. Jellis, G. S. Gray, J. Miller, and J. A. Bluestone, (1993), “Expression and Functional Significance of an Additional Ligand for CTLA-4,” Proc. Natl. Acad. Sci., USA, 90:11054–11058). In this case, long term donor specific tolerance was achieved.
3. Recombinant Phage Display Technology for Antibody Selection
To date, no monoclonal antibodies which crossreact with both B7.1 and B7.2 have been reported. Furthermore, no monoclonal antibodies which are specific to B7.1 or B7.2 and which also recognize specific sites on the antigens which are restricted to co-activation receptor CD28 binding have been reported. Or alternatively, no monoclonal antibodies which are specific to B7.1 or B7.2 and which recognize specific sites on the antigens which are exclusive of CTLA-4 receptor binding have been reported. As discussed supra, such antibodies would potentially be highly desirable as immunosuppressants.
Phage display technology is beginning to replace traditional methods for isolating antibodies generated during the immune response, because a much greater percentage of the immune repertoire can be assessed than is possible using traditional methods. This is in part due to PEG fusion inefficiency, chromosomal instability, and the large amount of tissue culture and screening associated with heterohybridoma production. Phage display technology, by contrast, relies on molecular techniques for potentially capturing the entire repertoire of immunoglobulin genes associated with the response to a given antigen.
This technique is described by Barbas et al, Proc. Natl. Acad. Sci., USA, 88, 7978–7982, (1991). Essentially, immunoglobulin heavy chain genes are PCR amplified and cloned into a vector containing the gene encoding the minor coat protein of the filamentous phage M13 in such a way that a heavy chain fusion protein is created. The heavy chain fusion protein is incorporated into the M13 phage particle together with the light chain genes as it assembles. Each recombinant phage contains, within its genome, the genes for a different antibody Fab molecule which it displays on its surface. Within these libraries, in excess of 106 different antibodies can be cloned and displayed. The phage library is panned on antigen coated microliter wells, non-specific phage are washed off, and antigen binding phage are eluted. The genome from the antigen-specific clones is isolated and the gene III is excised, so that antibody can be expressed in soluble Fab form for further characterization. Once a single Fab is selected as a potential therapeutic candidate, it may easily be converted to a whole antibody. A previously described expression system for converting Fab sequences to whole antibodies is IDEC's mammalian expression vector NEOSPLA. This vector contains either human gamma 1 or gamma 4 constant region genes. CHO cells are transfected with the NEOSPLA vectors and after amplification this vector system has been reported to provide very high expression levels (>30 pg/cell/day) can be achieved.
4. Primatized Antibodies
Another highly efficient means for generating recombinant antibodies is disclosed by Newman, (1992), Biotechnology, 10, 1455–1460. More particularly, this technique results in the generation of primatized antibodies which contain monkey variable domains and human constant sequences. This reference is incorporated by reference in its entirety herein. Moreover, this technique is also described in commonly assigned U.S. application Ser. No. 08/379,072, filed on Jan. 25, 1995, which is a continuation of U.S. Ser. No. 07/912,292, filed Jul. 10, 1992, which is a continuation-in-part of U.S. Ser. No. 07/856,281, filed Mar. 23, 1992, which is finally a continuation-in-part of U.S. Ser. No. 07/735,064, filed Jul. 25, 1991. 08/379,072 and the parent application thereof are incorporated by reference in their entirety herein.
This technique modifies antibodies such that they are not antigenically rejected upon administration in humans. This technique relies on immunization of cynomolgus monkeys with human antigens or receptors. This technique was developed to create high affinity monoclonal antibodies directed to human cell surface antigens.
Identification of macaque antibodies to human B7.1 and B7.2 by screening of phage display libraries or monkey heterohybridomas obtained using B lymphocytes from B7.1 and/or B7.2 immunized monkeys is also described in commonly assigned U.S. application Ser. No. 08/487,550, filed Jun. 7, 1995, incorporated by reference in its entirety herein. More specifically, 08/487,550 provides four monoclonal antibodies 7B6, 16C10, 7C10 and 20C9 which inhibit the B7:CD28 pathway and thereby function as effective immunosuppressants.
Antibodies generated in the manner described by these co-assigned applications have previously been reported to display human effector function, have reduced immunogenicity, and long serum half-life. The technology relies on the fact that despite the fact that cynomolgus monkeys are phylogenetically similar to humans, they still recognize many human proteins as foreign and therefore mount an immune response. Moreover, because the cynomolgus monkeys are phylogenetically close to humans, the antibodies generated in these monkeys have been discovered to have a high degree of amino acid homology to those produced in humans. Indeed, after sequencing macaque immunoglobulin light and heavy chain variable region genes, it was found that the sequence of each gene family was 85–98% homologous to its human counterpart (Newman et al, (1992), Id.). The first antibody generated in this way, an anti-CD4 antibody, was 91–92% homologous to the consensus sequence of human immunoglobulin framework regions. Newman et al, Biotechnology, 10:1458–1460, (1992).
Monoclonal antibodies specific to the human B7 antigen have been previously described in the literature. For example, Weyl et al, Hum. Immunol., 31(4), 271–276, (1991) describe epitope mapping of human monoclonal antibodies against HLA-B-27 using natural and mutated antigenic variants. Also, Toubert et al, Clin. Exp. Immunol., 82(1), 16–20, (1990) describe epitope mapping of an HLA-B27 monoclonal antibody that also reacts with a 35-KD bacterial outer membrane protein. Also, Valle et al, Immunol., 69(4), 531–535, (1990) describe a monoclonal antibody of the IgG1 subclass which recognizes the B7 antigen expressed in activated B cells and HTLV-1-transformed T cells. Further, Toubert et al, J. Immunol., 141(7), 2503–9, (1988) describe epitope mapping of HLA-B27 and HLA-B7 antigens using intradomain recombinants constructed by making hybrid genes between these two alleles in E. coli. 
High expression of B7 antigen has been correlated to autoimmune diseases by some researchers. For example, Ionesco-Tirgoviste et al, Med. Interre, 24(1), 11–17, (1986) report increased B7 antigen expression in type 1 insulin-dependent diabetes. Also, the involvement of B7 antigen expression on dermal dendritic cells obtained from psoriasis patients has been reported. (Nestle et al, J. Clin. Invest., 94(1), 202–209, (1994)).
Further, the inhibition of anti-HLA-B7 alloreactive CTL using affinity-purified soluble HLA-B7 has been reported in the literature. (Zavazava et al, Transplantation, 51(4), 838–42, (1991)). Further, the use of B7 receptor soluble ligand, CTLA-4-Ig to block B7 activity (See, e.g., Lenschow et al, Science, 257, 789, 7955 (1992)) in animal models and a B7.1-Ig fusion protein capable of inhibiting B7 has been reported.
Evidence is provided in this disclosure for the identification of monoclonal antibodies which recognize specific sites on the B7.1 antigen which are restricted to CD28 receptor binding. Furthermore, evidence is presented herein for the identification of antibodies which recognize sites on the B7.1 antigen which are exclusive of CTLA-4 receptor binding. Thus, evidence is presented herein to support the existence of unique antigen binding sites on the human B7.1 (CD80) co-stimulatory antigen. The sites claimed are identified by anti-B7.1 PRIMATIZED® antibodies and evidence is presented which confirms binding to a site of interaction on the B7.1 antigen which is restricted to binding with the co-activation receptor CD28.